Today many radio frequency (RF) receivers are super heterodyne receivers. FIG. 1 illustrates a RF receiver 100 that employs the heterodyne principle to down-convert and demodulate data from a RF signal. Generally, data are transmitted on a high frequency signal because of the intrinsic relationship between the RF's wavelength and the size of an antenna. The high frequency signal used to piggy back an information signal of lower frequency is called a carrier signal.
In an heterodyne system, a carrier signal is removed from a transmitted RF signal by mixing the received signal with another locally generated signal. The mixing process yields several signals at various frequency bands. The frequency band of interests is the intermediated frequency (IF) of the system, which contains data signals in modulated form. As illustrated in FIG. 1, RF signals received by an antenna are amplified and outputted to a mixer 110. The output of mixer 110 is inputted into an IF filter 120. IF filter 120 performs several important functions such as image rejection, amplification, and bandpass filtration. Depending upon the application, IF filter 120 may be a Bessel filter or more commonly a Butterworth filter. The latter is designed to provide a maximum frequency plateau of minimum ripple across the bandpass frequency of the filter. The former is designed to perform in the substantially the same way but with a time delay.
Currently there are several co-existing communication standards such as: global system for mobile communication (GSM), a second generation (2G) technology; universal mobile telecommunications system (UMTS), a third generation technology (3G) (UMTS is also known as wideband code division multiple access (W-CDMA)); enhanced data GSM environment (EDGE); and CDMA2000. Each standard typically operates at a different IF frequency and has a different bandwidth. Thus, each standard requires a different IF filter configuration.
One class of filters with a high frequency response is the transconductor capacitor (GmC) filter. FIG. 2 illustrates a conventional transconductor circuit 200 used to implement a GmC filter. Circuit 200 includes a pair of transistors 202 and 204, a pair of resistors 206 and 208, and a pair of current sources 210 and 212. The differential input voltages are received by the gates of transistors 202 and 204. In operation, transistor 202 outputs a current (Iout) when it is biased by a differential voltage (Vin+). The ratio of the output current and input voltage defines the transconductance (Gm) of transistor 202. Thus, the Gm of circuit 200 is:
      G    m    =            ∂              I        out                    ∂              V                  i          ⁢                                          ⁢          n                    
To increase the linearity of circuit 200, degenerative resistors 206 and 208 are coupled between the sources of transistors 202 and 204. Further, each source of transistors 202 and 204 is independently biased by current source 210 and 212. In this configuration, DC current flow through resistors 206 and 208 is not present and only AC current flow is allowed. This yields a transconductor with a better performance due to the elimination of voltage drop across the degenerative resistors.
As mentioned, each communication standard operates at a different IF frequency and bandwidth. Hence a receiver is typically designed to work optimally with a certain communication standard. For example, a GSM or EDGE compatible receiver must be configured to work with an IF signal with a center frequency of 200 KHz. For WCDMA, the same receiver must be configured to work with an IF signal with a center frequency of 600 KHz to 1000 KHz. Hence, in current receiver systems, a specific set of filters is designed and manufactured for each communication standard.
Accordingly, what is needed is a filter stage that can be implemented across various communication standards.